The invention relates to a method for the production of protective layers with a dark hue through pickling of rolled flat objects made of titanium zinc alloys, for use in the field of construction, whereby the flat objects are pickled in an aqueous pickling bath consisting of a mixed acid containing sulfuric acid and nitric acid, and are passivated after rinsing by forming a protective layer. The invention additionally relates to flat objects provided with protective layers.
Such a method is known from DE 196 36 370 A1 of the applicant, the subject matter of which is incorporated by reference in the present application. With the treatment of titanium zinc sheets made of a titanium zinc alloy with 0.07 to 0.12 percent by weight titanium, up to 0.015 percent by weight aluminum, 0.08 to 0.17 percent by weight copper and the remaining part of 99.995 percent zinc as disclosed in said document, homogeneous, gray protective layers are achieved on the surface of the titanium zinc sheets, whereby the gray coloration is perceived as particularly advantageous from an esthetic point of view because it corresponds to the natural patina of zinc. In its composition, the alloy disclosed in DE 196 36 370 A1 corresponds to the criteria of a “quality zinc” catalog of the “TÜV Produkt und Umwelt GmbH, Institut für Umweltschutz und Energietechnik”, located at Am grauen Stein, 51105 Köln.
However, it has become apparent that with the known method a darker coloration of the metal sheets, which is desired in some instances, can be achieved only to a limited degree, and only by using more strongly concentrated acids. This results in the problem that with stronger acid concentrations, i.e., lower pH values of the pickling bath, the substance of the alloy is attacked and the metal thickness is reduced. In particular, the use of a mixed acid with a very low pH value leads to an undesired layer structure on the surface because of the formation of poorly bonding layers of a so-called pickling sludge. Thus, with the known method, limits are set for a stronger darkening of the titanium zinc sheets. However, for the field of construction, often very dark colored surface layers are desired, in particular in the field of roof coverings or facade covers, in order to keep the contrast between the pre-weathered zinc sheets and other used construction materials, for example black slate shingles or pantiles, to a minimum, or to create a strong contrast to white surfaces.
Methods for creating a dark surface with titanium zinc through phosphating (cf. DIN 50 942) are known as well. However, these methods alter the metallic surface structure in an undesired manner if the surface is not to be coated with paint.